Best answer (Chosen by asker)
This can be put down to many different problems, but I will just cover the main problems. When you are copying on the fly, your computer has to read from one drive and then burn with the other. Sometimes the problem can be that the read speed is too slow for the burn speed. In some older versions of Nero, you could select a slower reading speed and the end result would probably be corrupt. Make sure your burn speed is higher than your read speed.
You may also want to defragment your HDD's every now and then. Running programs like Nero CD Speed and Drive Speed can also help Nero to learn more about your CD and DVD drives and will result in much less problems.
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